Vertical pressure type pulp screen



April 12, 1966 cow 3,245,535

VERTICAL PRESSURE TYPE PULP SCREEN le May 2 3 3 Sheets-Sheet 1 illlllilW Mimi 'allmT' mm- INVENTOR BE Cow/w ATTORNEYS A ril 12, 1966 B. OWAN3,245,535

VERTICAL PRESSURE TYPE PULP SCREEN Filed May 25, 1965 5 Sheets-Sheet 2INVENTOR BEN COWAN ATTORNEYS April 1966 B. cowAN 3,245,535

VERTICAL PRESSURE TYPE PULP SCREEN Filed May 23, 1963 5 Sheets-Sheet 3INVENTOR FYI-7.- El BEN Cow/m A TTORNE YS United States 3,245,535VERTECAL PRESSURE TYPE PULP SiIREEN Ben Cowan, 5460 Patricia Ave.,Montreal, Quebec, Canada Filed May 23, 1963, Ser. N 0. 2$2,666 4 Claims.(ill. 209240) This invention relates to rotary pulp screens of thevertical pressure type and particularly to a screen in which the rejectsand that portion of acceptable fibres carried with the rejects arepassed to a lower restricted annular area where the acceptable fibresand the rejects are separated from each other with the fibres beingpassed through the screen and the rejects are discharged through areject discharge chamber and through a reject discharge opening.

The conventional pressure or drowned type screen has been used recentlyin applications where it is necessary to prevent the entry of air intothe'pulp. In this case the screening operation is conducted under aslight pressure, in a completely closed system from feed'to discharge.

In the conventional pressure or drowned type of screen, the rejectmaterial remains in the inside of the screen plate and a current must beinduced along the face of the plate to move the rejects positively tothe discharge point. As the feed stock is the only fluid available tomove the rejects in this manner, a large amount of good fibre must berejected together with the true rejects, in order to discharge therejects from the screen. The usual practice is to rescreen the rejectsand their attached fibres over a coarse screen which accepts of thefibre back into the system. The net result is a screening whose cf".-ciency is determined by rejects screening and not the primary screening.

l in the present invention, in order to avoid the rejecting of a-fairlylarge quantity of fibre with the true rejects, the medium carrying therejects, i.e., the mixture of pulp and liquor, is displaced at'thebottom area of the screen by a clear liquor which is maintained at apressure in excess of that of the screen feed pressure. This clearliquor under pressure separates the pulp from the rejects and washes itthrough the screen and the rejects continue down, carried by the clearliquor, into a reject chamber and to the reject discharge outlet.

, The invention consists essentially in the provision of a verticallydisposed rotary impeller which is provided with a core in the form of ahollow frustrurn of a cone whose apex is pointed in the direction of thestock inlet of the screen. The impeller is provided with a series ofvertically disposed blade-s radiating outwards at right angles to theaxis of the impeller and extending along the surface of the impellercore and projecting upwards to the top level of the screen. A stationarycore formed from a frustrum of a cone, axially mounted about the driveshaft supporting the impeller, projects upwardly into the lower portionof the impeller core and is spaced radially inwards of the inner surfaceof the impeller core. A white water or clear liquor supply is connectedto an opening in the side of stationary core to direct an annular jetdownwards towards the annular reject chamber formed at the periphery ofthe large diameter end of the stationary core and also directs jetsthrough the apertures in the adjacent wall of the impeller core and intothe annular space between the lower end of the impeller core and theadjacent surface of the screen. It is in this annular space Where therejects are washed to separate the Patent 'ice fibres which are passedradially outwards through the screen while the rejects are carriedaxially downwards by the liquor flow into the annular reject chamber.

The conical section of the impeller core with the large end of the conelocated adjacent the reject discharge, provides a continually decreasingarea of flow axially towards the reject end of the screen and maintainsthe axial velocity of flow of the rejects along the inside of the screentowards the reject discharge. The axial section of the stationarycorerelative to the impeller core provides a continually decreasing areaof flow axially of the white water and ensures passage of the whitewater, or liquor under centrifugal force, through the apertures in theimpeller core to mix with and dilute the rejects in the constricted areabetween the impeller core and the inner surface of the screen and thusto separate the fibres still clinging to the rejects. Thus, in the areaof the screen adjacent the annular reject compartment, the dilutionliquor is used to wash away, through the screen, the acceptable fibresand the rejects are then discharged through the annular reject chamber.

The object of the invention is to provide a vertical type pulp screenoperating under pressure in which approximately all of the pulp fibresare screened from the reject material.

A further object of the invention is to provide a vertical type pulpscreen operating under pressure in which, in the final stage, themixture of pulp fibres and reject material remaining Within the screenis washed and separated by liquor other than that carrying the pulp intothe screen.

A further object of the invention is to provide a vertical type pulpscreen operating under pressure in which the impeller includes a conicalmember eflecting a continually decreasing area of flow of pulp stockaxially downwards towards the reject end of the machine.

A further object of the invention is to provide a vertical type pulpscreen in which a conical member will maintain the axial velocity offlow of the rejects along the inside of the screen plate towards thereject end of the machine.

A further object of the invention is to provide a vertical type pulpscreen in which the impeller includes a pair of conical members havingconcentric annular openings at their lower end aligned with an annularreject receiving chamber.

A further object of the invention is to provide a vertical type pulpscreen in which an outer conical member is provided with jet aperturesby means of which jets of white Water are directed on to the rejectmaterial to wash any fibres clinging to the rejects out through thescreen.

These and other objects of the invention will be apparent from thefollowing detailed description and the accompanying drawings showing apreferred embodiment of the invention, in which:

FIG. 1 is a vertical side elevation of the pulp screen looking on theinlet and discharge openings.

FIG. 2 is a horizontal section of the screen taken on the line 2-2 ofFIG. 1.

FIG. 3 is an enlarged vertical section of the screen taken on the line33 of FIG. 1.

FIG. 4 is a vertical elevation of the screen rotor partly in section.

FIG. 5 is a bottom plan view of the rotor shown in FIG. 4.

FIG. 6 is a vertical section of the fixed conical section of the machineshowing the reject discharge from the screen.

Referring to the drawings, the pulp screen 5 is of the vertical typeadapted to operate under pressure and consists of an outer circularcasing 6 whose wall is slightly tapered to provide a larger diameter atits lower discharge end than at its upper inlet end.

The casing 6 is closed at its top end by the plate 7 and at its lowerend by the annular plate 8 which supports on its under side the annularreject discharge chamber 9 to which the reject discharge outlet 10 isconnected. The base 11 of which the end plate 8 forms a part, supportsthe casing 6. The base 11 is in turn supported in an elevated positionby the foundation base 12.

The annular screen plate 13 has its lower end 14 secured to the plate 8at the outer periphery of the annular reject chamber 9.

A stationary core member 15 in the form of a frustrum of a cone has itsbase secured to the plate 8 at the inner peripheral edge of the annularreject discharge chamber 9. The upper end of the core member 15 isclosed 05 by a plate 16 having a centrally aligned seal ring 17, thepurpose of which will be explained later.

The upper end of the annular screen plate 13 is secured to the annularplate 13 which is secured to. the inner surface of the casing 6 at apredetermined distance down from the top of the casing to define aninlet chamber 1?. The plate 18 is supported in place by a series ofbrackets 20. The annular space 21 between the screen plate 13 and thecasing 6 defines a screened fibre chamber having a pulp fibre dischargeopening 22. A pulp fibre inlet 23 leads to the inlet chamber 19 of themachine.

The impeller 24 of the machine has a core 25 in the form of a frustrumof a cone whose larger diameter is located in the horizontal plane ofthe bottom end 26 of the impeller. The peripheral edge 27 of the largeend. of the cone 25 is such that when the impeller is axially alignedwith the core member 15 the peripheral edge 27 of the core member 25 islocated approximately midway of the radial width of the annular rejectchamber 9 (FIG. 3) and approximately a similar distance from theadjacent edge of the stationary core, member 15. The core member 25 isclosed off at the top by the plate 28. A plate 29 parallel with theplate 28 is located intermediate the height of the core 25. The plates28 and 29 are provided with axial apertures 30 and 31 and an axialsleeve 32 is located between these two plates.

A series of vertically disposed blades 33 are secured along their innervertical edges 34 to the outer surface of the core 25. The blades 33radiate outwards from the axis of the impeller 24. and each projectabove the top of the core 25 and are secured at their topmost edges 35to the annular ring 36. The top of the impeller, defined by the annularring 36 is approximately level with the plate 18 securing the top end ofthe screen plate 12 in the casing 6.

A series of nozzle apertures 37 are located in the tapered wall of theimpeller core 25 near its lower end and etween each of the impellerblades 33.

The impeller 2 is driven by means of the drive shaft 38 extendingupwards into the sleeve 32 and secured thereto by the key 39. The shaft33 is an extension of the drive sleeve 49 to which is secured the drivepulley 41 which, in turn, is driven by the motor 42 through the beltdrive 43. A drive shaft bearing assembly .4 is mounted between the upperand lower horizontal plates 8 and 45 of the screen supporting base 11 tosupport the drive shaft 38-40, in its vertically aligned position.Suitable belt tensioning means 46 are provided for tensioning the belt43.

The impeller 24 is secured against axial movement by the nut 47 threadedon the threaded extension 48 of the shaft 38.

A spring 4 located about the drive sleeve 4% and between the seal member17 on the core member 15 and the plate 29 of the impeller core member 25acts as a cushion between the stationary and moving elements of themachine. The seal member 17 provides a fluid seal between the stationaryand moving members of the screen.

A separate source of white liquor is fed through the pipe 59 and throughthe wall of the stationary core member 15 into the space 51. between thecore member 15 and the adjacent walls of the rotating core member 25 ofthe impeller 24.

An eyebolt S2 is provided for removal of the top cover plate 7, eyebolts53 provide for removal of the impeller 24- while eyebolts 54 provide forremoval of the stationary core 15.

In the operation of this invention the screen is designed to operateunder pressure i.e. with the interior of the screen being completelyfilled with pulp stock. The pulp stock is delivered to the inlet chamber19 at the top of the screen through the pulp inlet 23. From the chamber19 the pulp stock is directed downwards between the blades 33 and overthe surface of the conical core 25 in the direction of the arrows A,shown in FIG. 3, towards the interior surface of the screen plate 13. Inthe area between the impeller top ring 36 and the bottom of the screenplate 13, a large portion of the pulp fibres are washed through thescreen plate 13 and are discharged from the screen through the dischargeoutlet 22.

Because of the conical core member 25 of the impeller there is provideda continually decreasing area of flow axially of the pulp fibres towardsthe annular opening 55 from the interior of the screen into the annularreject chamber 9, and therefore there is maintained an axial velocity ofhow of the reject material along the inside of the screen plate 13. Atthe restricted passage leading to the annular opening 55, indicated bythe arrows B, the reject material is subjected to a separate washing byliquor fed through the pipe into the space 51. Here the liquor isprojected through the nozzle apertures 37 against the reject material tothoroughly wash and sep arate only pulp fibres still clinging to thereject material, forcing the pulp fibres through the screen plate 13 andinto the stream of pulp fibres being discharged through the pulp outlet22.

The heavier reject material falls down through the annular opening intothe reject chamber 9 and is dis-' charged through the discharge outlet10.

Suitable valves at the inlet and outlets of the screen (not shown)ensure that the screen is kept filled at all times.

By the use of the above described invention the axial flow of theheavier reject material is maintained downwards along the interior ofthe screen and through the rericted annular opening 55 into the annularchamber 9, and by the use of a separate source of washing liquor and notpulp stock, the reject material is thoroughly washed to the extent thatin actual operation less than 1% of the good fibre in the feed of thescreen may be discharged with the rejects, whereas this may be as highas 20% with conventional pressure type screens hitherto in use. Thebasic principle behind the design of this screen is the use, in a screenunder pressure, of an external liquid to displace the pulp medium usedto carry the tailings through the screen, and thereby to carry thetailings from the screen in a medium free from acceptable material. Thisgives a complete separation of acceptable and unacceptable material in asingle screen which is kept under pressure.

This result can be achieved presently in open type conventional screens,but cannot be achieved by any pressure type screen now in use.

The open type of screen cannot be used in certain applications whereexposure to air will cause entrainment of air bubbles in the liquidmedium with consequent production of foam and ill eifects on the pulp orpaper making process. This screen combines all the advantages ofconventional open screen with perfect separation and a closed screenwith no air entrainment.

Under certain conditions it may be desirable to operate the screenwithout the addition of dilution wash water at the bottom of the screen.In such case the valve 56 in the feed water pipe 50 is closed oif andthe unit operated as an ordinary pressure screen.

What I claim is:

1. A pulp screening machine of the vertical pressure operated type, thesaid machine including a base, a bearing vertically aligned in the saidbase, a drive shaft in said bearing and means to drive the said shaft, acasing axially mounted on said base with respect to the said driveshaft, a top closure plate for said casing, a pulp feed inlet to thesaid casing adjacent the top end thereof and a pulp discharge outletadjacent the lower end of said casing, an annular perforated screenplate within said casing and supported on said base, means to supportthe top end of said screen plate to said casing at a point adjacent thelowermost edge of the said pulp feed inlet and defining with said casinga pulp feed chamber above said screen plate, a rotary impeller axiallymounted on said drive shaft, the said impeller comprising a core memberin the form of a frustrum of a cone whose larger diameter is locatedimmediately above the said base and spaced inwardly of the said screenplate to define a restricted annular reject discharge opening whosesmaller diameter is locate-d a distance downwards from the top end ofthe said screen plate, a series of blades secured to the said coremember and radiating outwards therefrom, the said blades projectingupwards from said core member to the level of the top of said screenplate, an annular reject chamber in the said base whose outer peripherycoincides with the inner peripheral surface of the said screen plate andin communication with the interior thereof through said annular rejectdischarge opening, a stationary core member in the form of a frustum ofa core mounted on said base axially about said drive shaft andprojecting into the lower end of the said impeller core member andspaced radially inwards of the surface thereof to define a restrictedannular passage between the core stationary member and said impellercore member communicating with the said annular reject chamber, means tofeed a supply of washing liquor to the space between said stationarycore member and the impeller core member, and a series of apertures insaid impeller core member directing streams of washing liquor from thespace between the core members against the said screen plate adjacentthe said annular reject chamber.

2. A pulp screening machine of the vertical pressure operated type, thesaid machine including a vertically disposed casing having top andbottom closure plates, a pulp feed inlet to the casing adjacent its topend and a pulp discharge outlet to the casing adjacent its lower end, anannular perforated screen plate within said casing supported on the saidbottom closure plate, means to support the top end of the said screenplate at a point adjacent the lowermost edge of the said pulp feed inletand defining a pulp feed chamber above said screen plate, a rotaryimpeller axially mounted within said screen plate, the said impellercomprising a core member whose lower end is located in the horizontalplane of the lower end of said screen plate, the said core membereffecting a progressive reduction in area of the screening chamber in adownward direction and a series of vertically disposed blades secured tothe surface of said core member and radiating outwards therefrom towardssaid screen plate, the lower ends of said core member and said screenplate defining a first annular reject discharge outlet, a stationarycore member projecting upwardly from the said bottom closure platewithin the said impeller core member and defining therewith a washliquor pressure chamber, the lower ends of said impeller core member andstationary core member defining an annular wash liquor discharge outletconcentric with said first annular discharge outlet, and a rejectreceiving chamber in said bottom closure V 6 plate, the said rejectreceiving chamber communicating with the said screening chamber and washliquor pressure chamber through the said concentric annular dischargeoutlets.

3. A pulp screening machine of the vertical pressure operated type, thesaid machine including a vertically disposed casing having top andbottom closure plates, a pulp feed inlet to the casing adjacent its topend and a pulp discharge outlet to the casing adjacent its lower end, anannular perforated screen plate within said casing supported on the saidbottom closure plate, means to support the top end of the said screenplate at a point adjacent the lowermost edge of the said pulp feed inletand defining a pulp feed chamber above said screen plate, a rotaryimpeller axially mounted within said screen plate, the said impellercomprising a core member whose lower end is located in the horizontalplane of the lower end of said screen plate, the said core membereffecting a progressive reduction in area of the screening chamber in adownward direction and a series of vertically disposed blades secured tothe surface of said core member and radiating outwards therefrom towardssaid screen plate, the lower ends of said core member and said screenplate defining a first annular reject discharge outlet, a stationarycore member in the form of a frustum of a cone, projecting upwards fromthe said bottom closure and within the said impeller core member anddefining therewith a wash liquor pressure chamber, the larger diameterof the said stationary core member being located in the plane of thelower end of the said impeller core member and defining therewith anannular outlet from the said wash liquor pressure chamber concentricwith said first annular discharge outlet, and a reject receiving chamberin said bottom closure plate, the said reject receiving chambercommunicating with the said screening chamber and wash liquor pressurechamber through the said concentric annular discharge outlets.

4. A pulp screening machine of the vertical pressure operated type, thesaid machine including a vertically disposed casing having top andbottom closure plates, a pulp feed inlet to the casing adjacent its topend and a pulp discharge outlet to the casing adjacent its lower end, anannular perforated screen plate within said casing supported on the saidbottom closure plate, means to support the top end of the said screenplate at a point adjacent the lowermost edge of the said pulp feed inletand defining a pulp feed chamber above said screen plate, a rotaryimpeller axially mounted within said screen plate, the said impellercomprising a core member whose lower end is located in the horizontalplane of the lower end of said screen plate, the said core membereffecting aprogressive reduction in area of the screening chamber in adownward direction and a series of vertically disposed blades secured tothe surface of said core member and radiating outwards therefrom towardssaid screen plate, the lower ends of said core member and said screenplate defining a first annular reject discharge outlet, a stationarycore member projecting upwardly from the said bottom closure platewithin the said impeller core member and defining therewith a washliquor pressure chamber, a wash liquor feed pipe connected through saidstationary core member, feeding wash liquor into said wash liquorpressure chamber, a series of apertures in the said impeller coremember, the said apertures permitting the injection of wash liquor tobetween the said impeller and the screen plate adjacent the lower endthereof, the lower ends of the said impeller core member and stationarycore member defining an annular wash liquor discharge outlet concentricwith said first annular discharge outlet, and a reject receiving chamberin said bottom closure plate, the said reject receiving chambercommunicating with the said screening chamber and wash liquor pressurechamber through the said concentric discharge outlets.

(References on following page) References c115 by the Examiner F OREIGNPATENTS UNITED STATES PATENTS 115,639 1/1946 Sweden.

5,25% 5 HARRY THORNTON, Primary Examiner. 1:856:176 5/1932 Tfrimbf11:11:11: 209306 FRANK LUTTER, Examiner.

1,990,992 2/1935 Lang 210-415 R. HALF-ER, Assistant Examiner.

2. A PULP SCREENING MACHINE OF THE VERTICAL PRESSURE OPERATED TYPE, THESAID MACHINE INCLUDING A VERTICALLY DISPOSED CASING HAVING TOP ANDBOTTOM CLOSURE PLATES, A PULP FEED INLET TO THE CASING ADJACENT ITS TOPEND AND A PULP DISCHARGE OUTLET TO THE CASING ADJACENT ITS LOWER END, ANANNULAR PERFORATED SCREEN PLATE WITHIN SAID CASING SUPPORTED ON THE SAIDBOTTOM CLOSURE PLATE, MEANS TO SUPPORT THE TOP END OF THE SAID SCREENPLATE AT A POINT ADJACENT THE LOWERMOST EDGE OF THE SAID PULP FEED INLETAND DEFINING A PULP FEED CHAMBER ABOVE SAID SCREEN PLATE, A ROTARYIMPELLER AXIALLY MOUNTED WITHIN SAID SCREEN PLATE, THE SAID IMPELLERCOMPRISING A CORE MEMBER WHOSE LOWER END IS LOCATED IN THE HORIZONTALPLANE OF THE LOWER END OF SAID SCREEN PLATE, THE SAID CORE MEMBEREFFECTING A PROGRESSIVE REDUCTION IN AREA OF THE SCREENING CHAMBER IN ADOWNWARD DIRECTION AND A SERIES OF VERTICALLY DISPOSED BLADES SECURED TOTHE SURFACE OF SAID CORE MEMBER AND RADIATING OUTWARDS THEREFROM TOWARDSSAID SCREEN PLATE, THE LOWER ENDS OF SAID CORE MEMBER AND SAID SCREENPLATE DEFINING A FIRST ANNULAR REJECT DISCHARGE OUTLET, A STATIONARY